Why do you need a dust collection system? The most important reason is to avoid the health related risks. Inhaling fine dust can cause one to develop respiratory illnesses as well as aggravate existing respiratory conditions. Also, plastic dust on the floor is a slip hazard. Exposing employees to this type of environment will certainly result in workman’s compensation claims or even possibly a law suit. Providing dust collection at machinery as well as self-contained ceiling suspended dust collection units will keep your shop air virtually dust free.

Now that you have determined the need, you have to determine what size system is appropriate for your shop. If you have two or three machines and only use them a couple times a week you probably would buy one or two 2 h.p. portable dust collectors that can be rolled to each machine.

If you have a few more machines and you are using them simultaneously, even an hour or two per day, then you should consider installing a central dust collection system, which consists of a dust collector and piping. And there is only one way to pipe your system – metal pipe.

Plastic pipe systems are not designed for dust collection use. A necessary diversity of fittings to meet design requirements does not exist. Also, plastic pipe elbows have a very short radius and plastic tee fittings are improper for dust removal. It is these types of problems that lead to an inefficient dust collection system. Machining plastic will create a static situation and the waste wants to cling to anything (other than metal) especially plastic pipe.

Our recommendation is to install a metal piping system. With a metal dust collection piping system you do not have the concern of static electricity developing. Remember that metal is a conductor, plastic is a non-conductor. Elbows and other various fittings are properly designed for conveying dust. The diversity of fittings and accessories will enable you to meet design requirements. Ultimately, you will get the best performance from your dust collector with the proper metal ductwork.

Before you invest in a dust collector and ductwork, you need to size your system. All to often shop owners buy the collector first and either find that they do not have enough power or they spent too much money!

Before we explain how to design a basic system it is very important to understand that the following instructions and calculations are based on over thirty years of practical field experience. We have made some educated assumptions to make designing the system more understandable and easier than technical jargon.

The first step in designing your system is drawing a floor plan of your shop including location of dust producing machines (indicate size and location of dust pick-ups on each machine). Desired location of dust collector unit. Floor to joist measurements. And, finally any obstructions that would interfere with the run of the ductwork.

Some key terms you need to know before continuing are: CFM – Air volume in cubic feet per minute. FPM – Velocity of air in feet per minute. SP – Static Pressure: This is expressed in inches water gauge. It is the resistance to air in a duct, and is also commonly called: "resistance," "friction," "friction loss," or "pressure loss." VP – Velocity Pressure: expressed in inches water gauge. It is kinetic pressure in the direction of flow necessary to cause air at rest to flow at a given velocity.

Now you want to determine the velocity of your system, which is very easy. In order to convey plastic dust, the velocity is 4,000 FPM in branches and 3,500 FPM in the main. For plastic chips, the velocity is 4,500 FPM in branches and 4,000 FPM in the main.

Determining the size of the branches requires looking at each machine. There are actually several ways to determine the diameter of the branches. If the machine has a factory-installed collar, the manufacturer has determined that the machine needs that size branch under normal circumstances. If the machine has a metric diameter outlet, convert it into inches, and round off to the nearest inch (when writing up your parts list you may need to order a custom reducer). If the outlet is rectangular you need to determine the equivalent round diameter (when you write up your list use a transition).

Next you have to determine CFM requirements for each branch. See the CFM Chart below.

CFM CHART

Diameter

3,500 FPM

4,000 FPM

4,500 FPM

3 inch

170 CFM

195 CFM

220 CFM

4 inch

300 CFM

350 CFM

390 CFM

5 inch

475 CFM

550 CFM

610 CFM

6 inch

700 CFM

785 CFM

880 CFM

7 inch

900 CFM

1,100 CFM

1,200 CFM

8 inch

1,200 CFM

1,400 CFM

1,570 CFM

9 inch

1,550 CFM

1,800 CFM

1,990 CFM

10 inch

1,900 CFM

2,200 CFM

2,450 CFM

12 inch

2,800 CFM

3,100 CFM

3,600 CFM

14 inch

3,800 CFM

4,200 CFM

4,800 CFM

Determine which machines are your primary machines. A primary machine is the machine(s) that will operate at the same time under the worst conditions. (If you normally operate two machines, but once a week need to operate a third machine at the same time, then you must size your system for all three machines.) We generally highlight the primary machines on the drawing.

Size the Main Trunk Line. When sizing the main trunk line start with the primary machine farthest from the dust collector. Run that size duct until the next primary branch enters the main. Increase the main size at that junction to accommodate the CFM total of the two primaries. You will follow this practice all the way to the collector, sizing all primary junctions to accommodate total CFM of all primaries at that point. Do not increase main duct size when a branch other than a primary enters. Your total CFM requirement is the total of all primary branches. When not using a primary machine you will close the blastgate and divert suction to a secondary machine.

Example: You have 3 primary machines. You have already assigned the branch diameter and CFM requirements.

Table Saw, Router

4" Diameter

350 CFM

Panel Saw

5" Diameter

550 CFM

The main will be 4" from the table saw until the branch of the Panel Saw joins the main; at this point you need to increase your main to handle the combined CFM (350+550=900 CFM). Using the CFM Chart look up 900 CFM under the appropriate velocity (3,500 in the main), then look at the corresponding diameter (7"). You will run 7" pipe in the main from the Panel Saw until the branch of the Router joins the Main.

Here again you need to increase your main to handle the total CFM (900+350=1,250 CFM). Using the chart again you will see that 1,250 CFM is slightly more than volume for 8" diameter. Drop back to 8" diameter so as not to go below transport velocity. Run the 8" duct in your main from the Router to your Dust Collector. We always drop back from a half size duct to maintain transport velocity.

If you are installing an indoor re-circulating dust collector you need not calculate any more duct diameters. If you are attaching ductwork to the exhaust side of your dust collector it is accepted practice to use a duct diameter two diameters larger on the exhaust side than on the inlet side, thus minimizing exhaust and duct resistance.

Figure System Resistance (SP). The total static pressure is several factors added together. They are entry loss, dirty filter loss, static pressure of the worst branch duct, static pressure of main duct, and static pressure of the return duct.

There are more complicated ways to figure the entry loss of your system, but we find it usually equals a loss of 1" watergauge. (Use 1" as a constant). If your system has filters, add in a 2" loss. (If you do not have filters add zero). The Worst Branch, is the branch with the greatest resistance. The branch with the greatest resistance is usually a smaller diameter with the most lineal footage of pipe and elbows. Static pressure of worst branch and main duct can be calculated by using the following Static Pressure Chart

STATIC PRESSURE CHART(Static Pressure based on 100’ of Pipe)

Diameter

3,500 FPM

4,000 FPM

4,500 FPM

3 inch

7.5

10.0

12.0

4 inch

5.5

7.0

8.5

5 inch

4.2

5.5

6.5

6 inch

3.5

4.5

5.5

7 inch

2.6

3.8

4.5

8 inch

2.2

3.0

3.8

9 inch

2.0

3.0

3.4

10 inch

1.8

3.0

3.0

12 inch

1.2

1.8

2.5

14 inch

1.3

1.6

2.0

This Static Pressure Chart is based on 100 feet of pipe; therefore, you have to convert all elbows to an equivalent of pipe. To convert 90 and 45 degree elbows to equivalent feet of pipe use this Static Pressure Chart.

When figuring the feet of pipe count 45 degree lateral type branches as 45 degree elbows. Flexhose generally has 4 times the resistance as metal pipe. For this reason we suggest you keep hose to a minimum.

Example: A typical 4" branch consists of a lateral tee, a 45 degree elbow, a section of pipe, a 90 degree elbow and a blastgate.

Now using Static Pressure Chart read the friction loss (SP) under the corresponding Velocity and diameter pipe. You will see 100' of 4" pipe has a Static Pressure of 7" water gauge. To figure the static pressure of 22 feet multiply .22 by 7 which equals 1.54" water gauge. You don’t have to calculate static pressure for every branch, just the worst one. The static pressure of the main is done the same way, except you figure it out for each diameter in the main, starting farthest away and working toward the collector. Return duct resistance should also be calculated.

Now you have all the information you need to make an educated decision in purchasing your dust collector. You have determined the Velocity, CFM, Static Pressure and the size of the ductwork. Next go through the system, this time starting at the dust collector, and list each part you will need including spiral pipe, lateral tees, reducers, elbows, transitions, etc. Don't forget the assembly equipment such as pop rivets, hangers, strapping, caulking, and couplings. Make sure to tell your dust collector company what type of material you will be collecting. It doesn’t hurt to forward them a sample.

Written by Curt Corum, Technical Sales Manager, Air Handling Systems, which manufactures piping systems and accessories for dust collection systems.